Multivariate assessment of extraction conditions for the fractionation analysis of mercury in oily sludge samples using cold vapor atomic fluorescence spectrometry

Abstract

This work focused on the evaluation of extraction protocols using convective heating and HNO₃ or HCl solutions regarding their performance towards fractionation analysis of mercury (inorganic and organic fractions) in oily sludge samples using cold vapor atomic fluorescence spectrometry (CV-AFS). Sample mass, volume of extraction, temperature, type and concentration of the extractant solution (HNO₃ or HCl) and time of extraction were evaluated. Doehlert and Box–Behnken designs were employed to generate response surfaces that would allow multivariate evaluation of experimental conditions to be carried out. After statistical treatment and response surface evaluation, the conditions 100 mg of sample, 90 °C, 10 mL of 8.0 mol L⁻¹ HNO₃ and 10 minutes of extraction were selected and applied in a desirability function to combine the maximum extraction efficiency of Hg species and the least acceptable CH₃Hg⁺ conversion rate, as well as the highest recovery of Hg²⁺ and CH₃Hg⁺. Three different oily sludge samples were analyzed, resulting in 45–66% of Hg²⁺ fraction, negligible CH₃Hg⁺ to Hg²⁺ conversion and recoveries of 49–102% for Hg²⁺ and 89–106% for CH₃Hg⁺. The analytical results were compared to those obtained by high-performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and no statistical differences were observed at a 95% confidence level. Limits of detection (LOD) and quantification (LOQ) achieved for CV-AFS were 0.07 and 0.2 ng g⁻¹, respectively, whereas the LOD and LOQ achieved using HPLC-ICP-MS were 0.01 and 0.03 μg g⁻¹. The concentration of organic species of Hg in oily sludge samples was below the LOD of both techniques.